How does detergent change surface tension?

Thanks for your question about detergents and surface tension.

Water is a polar molecule. The molecules are highly attracted to each other, thus creating high surface tension (that's why people rarely survive falls into water from high bridges).

Detergents have two ends: one is polar and the other end is hydrophobic (water hating). The polar end of the detergent can bond with the polar water molecules, reducing the water's surface tension. This is an advantage when washing clothes because this can allow the dirt and grease to be removed by water with detergent. Water alone cleans poorly because the water molecules are attracted too much to each other.

Here is video showing how a paperclip can "float" on water - it's actually being held up by the hydrogen bonds formed between water molecules which give water its surface tension. Watch what happens when detergent is added to the beaker!!

video from: Noel Pauller

Here is a website where you can see how detergent will reduce surface tension of water. I love doing this experiment on the overhead projector. It's simple, fast, and the students can see what the detergent can do to reduce water's surface tension.
chemistry.about.com

Have a great day!!